CHAPTER 4

Analyzing Airport Terminal Area Roadways

This chapter presents an overview of terminal area roadway analyses. It presents level-of-service definitions applicable to airport roadways and describes methods for estimating the capacity and levels of service. Chapter 5 presents comparable methods for analyzing curbside roadways.

As described earlier, a hierarchy of analytical methods—including quick-estimation, macroscopic, and microsimulation methods—for analyzing airport terminal area roadway and weaving section operations is proposed. The appropriate analytical method will evolve as a project proceeds from concept to final design and as more time and data become available to support the analyses.

This chapter presents the suggested quick-estimation methods for analysis of airport roadways with uninterrupted flows, signalized roadways, and airport roadway weaving sections; the macroscopic method for analyzing low-speed roadway weaving areas commonly found on airports; and an overview of the use of microsimulation methods.

The macroscopic methods and performance measures presented in the HCM are considered applicable for analyses of airport roadways with uninterrupted traffic flows and unsignalized and signalized intersections, but not for analyses of low-speed roadway weaving areas. It is suggested that the method presented in the section “Macroscopic Method for Analyzing Airport Roadway Weaving Areas” be used when macroscopic analyses of airport weaving areas are required, and that the methods presented in HCM be used for macroscopic analyses of all other airport roadways. The methods in this chapter are based on those presented in the HCM. The latest edition of the HCM should be used for analysis, recognizing that judgment may be needed in adapting subsequent updates to the HCM to airport conditions.

The methods and data presented in this chapter represent the best available information concerning airport roadway operations and the consensus of the research team, the project panel, and other reviewers at the time this Guide was prepared. It is suggested that additional research be conducted on low-speed weaving areas and maximum service rates for airport roadways.

4.1 Quality-of-Service Definitions for Airport Terminal Area Roadways

The key performance measures defining the quality of service of an airport terminal area roadway, including its sufficiency, are as follows:

• Average travel speed of through vehicles, which determines travel time.
• Traffic density, which determines the ability of motorists to easily maneuver into and out of travel lanes.
• Maximum volume-to-capacity (v/c) ratio, which along with through movement travel speed, indicates how close the roadway is to breakdown and is useful for determining other performance measures such as queue length and delays.

• Duration of congestion and length of queues.
• Travel-time reliability reflecting the distribution of trip times over an extended period.

With the exception of the weaving analysis discussed in this chapter, the definitions, metrics, and procedures presented in the HCM are applicable to airport roadways with uninterrupted operations (e.g., freeways and other grade-separated facilities) and roadways that are governed by intersections (e.g., signalized intersections, stop-controlled intersections, and roundabouts).

The weaving analysis methods presented in the HCM method are primarily oriented toward operations on freeways and were developed using data primarily from freeways. At airports, weaving often takes place on roadway segments designed for speeds that are much slower than those on freeways or even on major arterial streets. As a result, while the weaving theory and methods presented in the HCM (and subsequent updates) may be applicable to airport roadways, the metrics defining quality of service do not apply. Consequently, subsequent portions of this chapter present alternative metrics for the low-speed weaving that occurs on airport roadways.

4.2 Quick-Estimation Methods for Analyzing Airport Roadway Operations

This section presents quick-estimation methods for analyzing uninterrupted flows, signalized roadways, and airport roadway weaving sections.

4.2.1 Quick-Estimation Method for Uninterrupted Flows on Airport Roadways

Quick-estimation methods are most appropriate for “sizing” a roadway in the early stages of planning and the design process when little has been decided (or is known) about the details of the required roadway. Such methods are suitable for use when preparing planning studies to size or evaluate a roadway and identify points of existing or future constraints.

Table 4-1, which is adapted from the HCM (Exhibits 12-16 and 12-18), presents the maximum service flow rate and adjusted flow rates for multilane roadways with uninterrupted flows. The adjusted flow rates represent the maximum flow rates of typical airport access and circulation roadways and were calculated assuming that (1) heavy vehicles—primarily single-unit trucks and courtesy vehicles—comprise less than 15% of the traffic volume on the access roadways and (2) a high proportion of drivers are infrequent users of the airport roadways and are likely to be unfamiliar with them. The free-flow speeds can be approximated by the posted speed limits on the roadway section unless drivers regularly exceed the posted speed limit, in which case the free-flow speed can be approximated by the average operating speed of the vehicles on the roadway.

These adjusted flow rates are also based on the following assumptions:

• Travel lanes are at least 12 feet wide.
• Lateral clearances (e.g., distance from walls, abutments, or other physical obstacles) are at least 6 feet on both the left and right sides of the roadway.
• Any vertical grades are less than 0.25 mile in length or less than 3% (i.e., rises less than 3 feet per every 100 feet of length).
• The roadways operate in one direction only, or for two-way roadways, at least two travel lanes are provided in each direction, separated by a median.
• There are fewer than 10 driveways or other access points per mile along the airport roadway.

If the roadway being evaluated falls significantly outside the lane width, lateral clearance, and percent of truck use, and varies from the other factors listed above, then the traffic volume thresholds presented in Table 4-1 may not be accurate. In addition, given that airport roadways

Table 4-1. Levels of service for airport terminal area access and circulation roadways.

may experience a high share of infrequent users, all lanes on a given roadway segment may not be used equally. For example, if on a multilane roadway, signs indicate that certain facilities will be on the left, drivers may shift into the left lane well in advance of the actual exit. As a result, the combined lanes of the roadway may not achieve their full capacity or maximum service flow rate because many drivers would be unwilling to use each available lane. In such situations, a more detailed macroscopic analysis using procedures described in the HCM may be necessary to determine the maximum service volume for the facility.

If the lane width, lateral clearance, percent of truck use, and other factors described above are applicable to the roadway being analyzed, then the information in Table 4-1 should be applied as follows:

1. Determine the free-flow speed for the roadway. The free-flow speed is usually determined by measuring the mean speed of traffic under very light flow conditions. However, the posted speed limit can be used as an approximation of the free-flow speed.
2. Determine the target roadway sufficiency. The target is determined by individual airport operators (or local agencies) reflecting their individual policies and standards. If such a

2. standard or policy is lacking, “near capacity” (equivalent to LOS D in the HCM) is a common standard for urban roadways, although many urban agencies have adopted “at capacity” (equivalent to LOS E in the HCM) as a standard. “Below capacity” (equivalent to LOS C in the HCM) is considered the common standard for planning new airport facilities, while at large-hub airports, “near capacity” is sometimes considered to be acceptable on existing roadways during peak periods.
3. Using Table 4-1, select the appropriate free-flow speed and the column with the desired level of service. The maximum flow provides the maximum traffic per hour per lane that the roadway can serve in one direction.

For example, if the free-flow speed is 50 mph and the target roadway sufficiency is “near capacity,” then the maximum desirable flow rate for a two-lane, one-way road would be 3,040 vehicles per hour (twice 1,520).

4.2.2 Quick-Estimation Method for Signalized Roadways

The HCM (Volume 4, Chapter 31) presents a planning-level analysis intended to support simplified and approximate analyses of signalized intersections for motorized vehicles. This planning-level analysis has its roots in critical movement analysis. This method involves the following steps:

• Step 1: Identify the lane geometry.
• Step 2: Identify the hourly volumes, including left-turn, through, and right-turn volumes for each intersection approach.
• Step 3: Identify the signal phasing (i.e., which movements operate concurrently).
• Step 4: Perform left-turn check to determine the probability of each critical approach volume clearing the identified opposing or conflicting left-turn volume.
• Step 5: Assign lane volumes.
• Step 6: Identify critical volumes by identifying the conflicting or opposing traffic volumes (on a per lane basis) having the highest total volumes for each signal phase.
• Step 7: Sum the critical volumes.
• Step 8: Determine the intersection level of service.

4.3 Macroscopic Method for Analyzing Airport Roadway Weaving Areas

The HCM provides methodologies for evaluating traffic operations on airport roadways. However, the HCM methodologies were not designed to evaluate weaving conditions for low-speed airport roadways (speed limits of 30 mph or slower). These limits on applicability are commonly included in software that applies the HCM method, prohibiting the user from applying the software to weaving sections with free-flow speeds lower than 35 mph.

Consequently, a separate weaving analysis without the limitation on low free-flow speeds was developed and incorporated into a macroscopic model—QATAR. QATAR includes components that provide information about low-speed weaving and curbside roadway operations given certain inputs. The low-speed weaving operations are described in this section. The curbside operations components are described in Chapter 5.

QATAR uses the weaving analysis calculations and methodology presented in Chapter 13 of the HCM, for one-sided and two-sided weaving and applies these calculations to roadways having free-flow speeds slower than the lower bound of speeds (free-flow speeds less than 35 mph) presented in the HCM, 6th Edition. Three modifications were made to the HCM weaving method to extend its application to lower-speed roadway sections. First, the minimum speed for

weaving traffic was reduced from 15 mph in the HCM to 10 mph. Second, the HCM level-of-service thresholds were converted to sufficiency thresholds for consistency with other recommended analyses on airport roadways, and a special set of sufficiency threshold traffic densities was developed for application to weaving sections on low-speed airport roadways. Third, as an input in determining the capacity of the weaving segment, maximum service flow rates for basic freeway segments under base conditions were extrapolated to correspond to input free-flow speeds (i.e., less than 55 mph).

The HCM presents macroscopic methods for analyzing airport roadway operations away from the terminal area. These methods, if adjusted for the factors used to develop Table 4-1 (e.g., heavy vehicles and roadway geometry), are applicable to analysis of airport roadways with uninterrupted traffic flows and flows at signalized intersections, roundabouts, and stop-controlled intersections.

4.3.1 Use of HCM Weaving Analysis Procedure

The HCM weaving analysis procedure involves the following seven steps, which are described in this section:

• Step 1: Collect and input roadway weaving section lane geometry, lane designations, free-flow speed, and peak-hour volumes.
• Step 2: Adjust the mixed flow traffic volumes to equivalent passenger-car volumes (adjust for percent of heavy vehicles, driver familiarity, and peak-hour factor).
• Step 3: Determine configuration characteristic, which is based on lane changes of weaving movements.
• Step 4: Determine the maximum weaving length, if weaving analysis is appropriate.
• Step 5: Determine the weaving section capacity.
• Step 6: Determine lane-changing rates.
• Step 7: Determine the average speed of weaving and non-weaving vehicles.

An eighth step in the HCM, determining the level of service, has been replaced in this report with determining the sufficiency of the weaving segment.

The remaining paragraphs of this section describe these steps in more detail with the recommended modifications for applying this analysis to weaving sections of low-speed airport roadways. Equations and additional detail on these steps are provided in the HCM.

Collect and Input Data

The analyst must collect data on existing and/or forecast peak-hour traffic volumes for each leg of the weaving section. The traffic data should include a peak-hour factor and percent of heavy vehicles. The peak-hour factor is the ratio of the total peak-hour flow rate (in vehicles per hour, vph) divided by the peak 15-minute flow rate within the peak hour (converted to vph).

The free-flow speed or posted speed limit should be observed (or estimated in the case of a new design or planning study).

The proposed (or existing) lane geometry must be identified (number of lanes on each leg, number of lanes in the weaving section, lane striping showing how the lanes on each leg transition to and from the lanes in the weaving section, and the length of the weaving section).

Adjust Flow Rates

Mixed (passenger cars, trucks, buses, etc.) flow rates should be converted to equivalent passenger-car rates by accounting for a peak-hour factor and the presence of heavy vehicles. The HCM

no longer includes a driver familiarity factor, and it combines all heavy vehicle types into a single classification.

The user has two options for entering traffic volumes through the weaving segment. The first option is to enter actual origin and destination counts (or projected volumes) on the weaving section, and the second option is to enter approach and departure volumes, and then use QATAR to estimate the weaving volumes in the segment.

Determine Weaving Configuration

Several key parameters characterize the configuration of a weaving segment. The first step is to determine whether the roadway being analyzed is a one-sided ramp weave or a two-sided weave (graphical illustrations are provided in QATAR as well as in Figure 4-1; a photo of a weaving configuration is provided as Figure 4-2).

The key variables in subsequent steps of the methodology, for both types of weaving configurations are

For a one-sided weaving segment, the two weaving movements are the ramp-to-freeway and freeway-to-ramp flows. For the purposes of this analysis, the “freeway” flows are the mainline flows, and the “ramp” flows are those flows that merge into or diverge from the mainline flow. The following values are established:

For a two-sided weaving segment, only the ramp-to-ramp movement is functionally “weaving.” The following values are established:

Determine Maximum Weaving Length

The concept of “maximum length” (L_MAX) of a weaving segment is critical to the methodology. Strictly defined, the maximum length is the length beyond which weaving turbulence no longer affects operations within the segment, or alternatively, no longer affects the capacity of the weaving segment.

If the length of the weaving segment is greater than or equal to L_MAX, then this weaving analysis methodology is not appropriate, and the weaving segment should be analyzed as separate merge, diverge, and basic segments as appropriate.

Determine Capacity of Weaving Segment

Weaving capacity is determined by two methods: density and weaving demand flows. The final capacity is the smaller of the results of the two methods.

Determine Lane-Change Rates

The equivalent hourly rate at which weaving and non-weaving vehicles make lane changes within the weaving segment is a direct measure of turbulence in the flow of traffic (i.e., when vehicles exhibit irregular and apparently random fluctuations in speed). It is also a key determinant of speeds and densities within the segment, which ultimately determine the existing or anticipated level of service.

Determine Average Speeds of Weaving and Non-Weaving Vehicles in Weaving Segment

The methodology is used to compute the average speed of weaving vehicles in a weaving segment and the average speed of non-weaving vehicles in a weaving segment. For airport roadways, a minimum average speed (in miles per hour) of weaving vehicles expected in the weaving segment of S_MIN = 10 mph is recommended.

Note that, usually, the non-weaving speed should be modestly faster than the weaving speed. However, the developers of the HCM weaving methodology believe that it is acceptable for the non-weaving speed to be slightly slower than the weaving speed in some cases. If the analyst finds that the non-weaving speed is more than 3 mph to 5 mph below that of the weaving speed, then it is recommended that the analyst recompute the weaving speed using a lower S_MIN of 5 mph (instead of 10 mph). The analyst should be aware that these extrapolations of the HCM weaving methodology to airport roadways are very approximate; field conditions should be verified where possible.

The average speed of all vehicles in a weaving segment is computed using the average speeds for weaving and non-weaving vehicles.

Determine Sufficiency

The sufficiency of a weaving segment is related to the density in the segment. This is similar in concept to the use of level of service in the HCM for freeway segments.

Table 4-2. Sufficiency criteria for weaving segments.

Source: Adapted from HCM, Exhibit 13-6.

Notes: pc/mi/ln = passenger cars per mile per lane.
If the density exceeds the level-of-service threshold, then the roadway is over capacity.

Density is used to look up sufficiency in Table 4-2. A special set of density thresholds has been developed for weaving on low-speed airport roadways. Airport operators may choose their own thresholds based on local experience and perceptions of quality of service.

4.3.2 Caveats

Without more extensive research, it is impossible to know with certainty whether or not the results of the low-speed weaving macroscopic model presented in this section are accurate, but the results can provide an initial indication of whether a weaving section with certain parameters might operate successfully or not.

The results of the low-speed weaving analysis method and the revised metrics appear to (1) correlate reasonably well with the observations of airport roadway weaving operations conducted as part of the original ACRP Report 40 research, and (2) produce results suitable for planning-level analyses of low-speed airport roadway weaving operations. While low speeds can be entered as inputs to most microsimulation models, it is not known whether the resulting modeled traffic flows represent actual traffic operation patterns under those conditions. Few, if any, studies have been conducted on the low-speed weaving conditions typical of airport roadways to allow full verification of the suggested low-speed weaving analysis method outputs. Significantly more observations at numerous locations are required to provide a basis for analysis of low-speed roadway weaving operations that is consistent with the level of analytical precision of the HCM or any similar document.

The proposed low-speed weaving method is not intended to serve as a basis for any of the following:

• Design of a new low-speed roadway weaving section,
• Design of modifications to an existing low-speed weaving section,
• A definitive operational analysis of an existing or proposed weaving section, or
• The assessment of the level of safety afforded by an existing roadway.

Under the above conditions, microsimulation models may be more appropriate for evaluating traffic operations.

4.4 Use of Microsimulation Methods

Microsimulation modeling is an analytical process that uses sophisticated computer programs to analyze traffic operations for complex roadway systems and support the design of roadway facilities. In microsimulation modeling, individual simulated vehicles are assigned characteristics, such as a destination, vehicle performance capabilities, and driver behavioral profiles. Each “vehicle” then travels through a computerized roadway network, and various aspects of its performance are recorded during its simulated trip based on its interactions with other vehicles and

traffic controls. These performance statistics can be summarized in many ways, including performance measures commonly used by traffic engineers and transportation planners (e.g., delays, travel times, travel speeds, and queue lengths).

Some aspects of roadway systems, such as intersections controlled by isolated or coordinated traffic signals, can be analyzed using simpler techniques than microsimulation. The use of microsimulation models can be beneficial in other roadway environments, including those with complex traffic movements, such as weaving operations where some vehicles are entering, some are exiting, and some are traveling through the weaving sections.

Many airport roadway systems are sufficiently complex to warrant the use of microsimulation. The use of microsimulation models should be considered if simpler analytical tools and methodologies do not yield reasonable results, provide sufficient detail, or cannot be used because the roadway configuration or operating conditions are outside the range of those addressed in the HCM. However, the use of microsimulation models and analyses of traffic using these models is relatively complex. Training in the use of the specific model and experience in traffic engineering are required to fully understand the simulation process so that appropriate inputs are used and the outputs are interpreted correctly. Most microsimulation software packages require significant time to learn, as well. In addition, microsimulation requires input data that may be unavailable for the airport roadway to be analyzed, or the available data may not be sufficiently accurate or reliable to justify the effort required to develop and calibrate the model.

Suggested guidelines on when microsimulation is probably not needed are as follows:

1. Signalized or unsignalized intersections. These can usually be analyzed using methodologies in the HCM (Chapter 31). If exclusive turn lane storage area overflows, through movement queues that block access to exclusive turn lanes, or both are a significant problem, then HCM methodologies may yield optimistic estimates of signal performance. In this case, microsimulation modeling may yield more accurate results.
2. Roadway segments having few or no driveways or intersecting side roads.
3. Weaving segments with two entries and two exits and a reasonable distance (e.g., at least 500 feet) between the entrance to and exit from the segment and free-flow speeds of 35 mph or greater.
4. If the use of a simpler technique—even if the inputs are outside of the recommended ranges—yields outputs that are consistent with observed conditions (e.g., traffic seems very congested, and use of the HCM methodology yields LOS E or F).
5. Cases where inputs have a high degree of uncertainty, such as when future forecast volumes are at best approximate. In these cases, the apparent precision of microsimulation may not produce results that are any more accurate than simpler techniques that can be used with less time and effort.

Guidelines regarding when to consider microsimulation:

1. Signalized or unsignalized intersections that have more than four legs or are oriented in atypical ways. The analyst should initially attempt to use HCM methodologies and then consider microsimulation modeling if the inputs required to evaluate the roadway segments do not correspond to the HCM analysis structure.
2. Airport roadway segments with the number of lanes changing along the length of the segment and with multiple, and possibly unusual, orientations of driveways or intersecting side roads.
3. Multilane airport roadway segments where distribution of vehicles between the lanes is strongly influenced by which lane serves exits to downstream destinations.
4. If the access or circulation roadways contain roundabouts. The HCM provides guidelines on the analysis of roundabouts.

5. Weaving segments that do not fit the orientation of the weaving analysis in the HCM. This could mean more than two entries or exits, dimensions or speeds outside the bounds defined in the HCM, signals or stop signs within the weaving segment, etc.
6. If simpler techniques are used to analyze what appear to be sufficiently simple facilities, but the results indicate operations that are much worse or much better than those observed.
7. When congestion on one roadway section causes queues or backups that extend back and interfere with operations on an upstream critical roadway section.
8. When congestion on one roadway section significantly restricts the volume of vehicles that can arrive at a downstream critical location.
9. When comparing the congestion resulting from different improvement options for situations where it is not possible to design sufficient capacity to eliminate significant congestion. In this case, comparisons are typically made of the extent of the congestion (duration and length of queues) produced by the various improvement options. Microsimulation modeling is the best available tool for making these comparisons.

FHWA’s Traffic Analysis Toolbox III: Guidelines for Applying Traffic Microsimulation Modeling Software 2019 Update to the 2004 Version (Publication Number FHWA-HOP-18-036, April 2019) provides additional information on the use and application of microsimulation. This document is available at https://ops.fhwa.dot.gov/publications/fhwahop18036/index.htm.

4.5 Other Performance Measures

At some airports, the adequacy of a roadway or curbside area has been defined by the length of time a motorist requires to enter and exit the terminal area. Microsimulation models can be used to establish a baseline condition and compare the baseline travel time (or a predetermined acceptable travel time) with the travel times resulting from different levels of traffic demand and access and circulation roadway configurations. However, it is difficult to accurately estimate these travel times and queues without the aid of microsimulation models because of the relatively short distances being analyzed and the difficulty in estimating queue lengths through other means.